Ataxia-telangiectasia: founder effect among north African Jews

A genetic profile of contemporary Jewish populations

The Jews are an ancient people with a history spanning several millennia. Genetic studies over the past 50 years have shed light on Jewish origins, the relatedness of Jewish communities and the genetic basis of Mendelian disorders among Jewish peoples. In turn, these observations have been used to develop genetic testing programmes and, more recently, to attempt to discover new genes for susceptibility to common diseases.

The Ataxia telangiectasia gene product is required for oxidative stress-induced G1 and G2 checkpoint function in human fibroblasts

Ataxia telangiectasia (AT) is an autosomal recessive disorder characterized by neuronal degeneration accompanied by ataxia, telangiectasias, acute cancer predisposition, and sensitivity to ionizing radiation (IR). Cells from individuals with AT show unusual sensitivity to IR, severely attenuated cell cycle checkpoint functions, and poor p53 induction in response to IR compared with normal human fibroblasts (NHFs). The gene mutated in AT (ATM) has been cloned, and its product, pATM, has IR-inducible kinase activity. The AT phenotype has been suggested to be a consequence, at least in part, of an inability to respond appropriately to oxidative damage. To test this hypothesis, we examined the ability of NHFs and AT dermal fibroblasts to respond to t-butyl hydroperoxide and IR treatment. AT fibroblasts exhibit, in comparison to NHFs, increased sensitivity to the toxicity of t-butyl hydroperoxide, as measured by colony-forming efficiency assays. Unlike NHFs, AT fibroblasts fail to show G(1) and G(2) phase checkpoint functions or to induce p53 in response to t-butyl hydroperoxide. Treatment of NHFs with t-butyl hydroperoxide activates pATM-associated kinase activity. Our results indicate that pATM is involved in responding to certain aspects of oxidative damage and in signaling this information to downstream effectors of the cell cycle checkpoint functions. Our data further suggest that some of the pathologies seen in AT could arise as a consequence of an inability to respond normally to oxidative damage.

The founder mutations in the BRCA1, BRCA2, and ATM genes in Moroccan Jewish women with breast cancer

To gain insight into the molecular mechanisms underlying the inherited predisposition to breast cancer in non-Ashkenazi Jews, we genotyped 54 Jewish Moroccan women with breast cancer, unselected for family history of cancer, for the predominant Jewish mutations in BRCA1, BRCA2, and ATM. One patient (2%) was found to have the 185de1AG BRCA1 mutation, none was a carrier of the 6174delT BRCA2 mutation, and 2/54 (4%) were heterozygous for the ATM mutation. These rates were not significantly different from the rates in the general non-Ashkenazi population. These preliminary data may indicate that the predominant Jewish mutations in BRCA1, BRCA2, and ATM genes contribute little, if any, to breast cancer predisposition and risk among Moroccan Jews.

Characterization of ATM mutations in 41 Nordic families with ataxia telangiectasia

The Ataxia Telangiectasia Mutation (ATM) gene is mutated in the rare recessive syndrome Ataxia Telangiectasia (AT), which is characterized by cerebellar degeneration, immunodeficiency, and cancer predisposition. In this study, 41 AT families from Denmark, Finland, Norway, and Sweden were screened for ATM mutations. The protein truncation test (PTT), fragment length and heteroduplex analyses of large (0.8-1.2 kb) cDNA fragments were used. In total, 67 of 82 (82%) of the disease-causing alleles were characterized. Thirty-seven unique mutations were detected of which 25 have not previously been reported. The mutations had five different consequences for the ATM transcript: mutations affecting splicing (43%); frameshift mutations (32%); nonsense mutations (16%); small in-frame deletions (5%); and one double substitution (3%). In 28 of the probands mutations were found in both alleles, in 11 of the probands only one mutated allele was detected, and no mutations were detected in two Finnish probands. One-third of the probands (13) were homozygous, whereas the majority of the probands (26) were compound heterozygote with at least one identified allele. Ten alleles were found more than once; one Norwegian founder mutation constituted 57% of the Norwegian alleles. Several sequence variants were identified, none of them likely to be disease-causing. Some of them even involved partial skipping of exons, leading to subsequent truncation of the ATM protein.

Utilization of oriented peptide libraries to identify substrate motifs selected by ATM

The ataxia telangiectasia mutated (ATM) gene encodes a serine/threonine protein kinase that plays a critical role in genomic surveillance and development. Here, we use a peptide library approach to define the in vitro substrate specificity of ATM kinase activity. The peptide library analysis identified an optimal sequence with a central core motif of LSQE that is preferentially phosphorylated by ATM. The contributions of the amino acids surrounding serine in the LSQE motif were assessed by utilizing specific peptide libraries or individual peptide substrates. All amino acids comprising the LSQE sequence were critical for maximum peptide substrate suitability for ATM. The DNA-dependent protein kinase (DNA-PK), a Ser/Thr kinase related to ATM and important in DNA repair, was compared with ATM in terms of peptide substrate selectivity. DNA-PK was found to be unique in its preference of neighboring amino acids to the phosphorylated serine. Peptide library analyses defined a preferred amino acid motif for ATM that permits clear distinctions between ATM and DNA-PK kinase activity. Data base searches using the library-derived ATM sequence identified previously characterized substrates of ATM, as well as novel candidate substrate targets that may function downstream in ATM-directed signaling pathways.

ATM mutations in patients with ataxia telangiectasia screened by a hierarchical strategy

ATM has been identified as a gene that is responsible for ataxia telangiectasia (AT), a pleiotropic disorder of autosomal recessive inheritance. While many mutations of this gene in AT patients of various ethnicities have been reported, data on Japanese patients are scarce. In this report, we present the results of a thorough survey of ATM mutations in 14 unrelated AT patients, with an emphasis on Japanese subjects. We used a hierarchical strategy in which we extensively analyzed the entire coding region of the cDNA. In the first stage, point mutations were sought by PCR-SSCP in short patches. In the second and third stages, the products of medium- and long-patch PCR, each covering the entire region, were examined by agarose gel electrophoresis to search for length changes. We found a total of 15 mutations (including 12 new) and 4 polymorphisms. Abnormal splicing of ATM was frequent among Japanese, and no hotspot was obvious, suggesting no strong founder effects in this ethnic group. Eleven patients carried either one homozygous or two compound heterozygous mutations, one patient carried only one detectable heterozygous mutation, and no mutation was found in two patients. Overall, mutations were found in at least 75% of the different ATM alleles examined. Possible reasons for the inability to detect mutations in some patients are discussed.

ATM germline mutations in classical ataxia-telangiectasia patients in the Dutch population

Germline mutations in the ATM gene are responsible for the autosomal recessive disorder ataxia-telangiectasia (A-T). In our study, we have determined the ATM mutation spectrum in 19 classical A-T patients, including some immigrant populations, as well as 12 of Dutch ethnic origin. Both the protein truncation test (PTT) and the restriction endonuclease fingerprinting (REF) method were used and compared for their detection efficiency, identifying 76% and 60% of the mutations, respectively. Most patients were found to be compound heterozygote. Seventeen mutations were distinct, of which 10 were not reported previously. Mutations are small deletions or point mutations frequently affecting splice sites. Moreover, a 16.7-kb genomic deletion of the 3' end of the gene, most likely a result of recombination between two LINE elements, was identified. The most frequently found mutation, identified in three unrelated Turkish A-T individuals, was previously described to be a Turkish A-T founder mutation. The presence of a founder mutation among relatively small ethnic population groups in Western Europe could indicate a high carrier frequency in such communities. In patients of Dutch ethnic origin, however, no significant founder effect could be identified. The observed genetic heterogeneity including the relative high percentage of splice-site mutations had no reflection on the phenotype. All patients manifested classical A-T and increased cellular radioresistant DNA synthesis.

Identification of ATM mutations using extended RT-PCR and restriction endonuclease fingerprinting, and elucidation of the repertoire of A-T mutations in Israel

Ataxia-telangiectasia (A-T) is an autosomal recessive disorder characterized by neurodegeneration, immunodeficiency, cancer predisposition, and radiation sensitivity. The responsible gene, ATM, has an extensive genomic structure and encodes a large transcript with a 9.2 kb open reading frame (ORF). A-T mutations are extremely variable and most of them are private. We streamlined a high throughput protocol for the search for ATM mutations. The entire ATM ORF is amplified in a single RT-PCR step requiring a minimal amount of RNA. The product can serve for numerous nested PCRs in which overlapping portions of the ORF are further amplified and subjected to restriction endonuclease fingerprinting (REF) analysis. Splicing errors are readily detectable during the initial amplification of each portion. Using this protocol, we identified 5 novel A-T mutations and completed the elucidation of the molecular basis of A-T in the Israeli population.

Diversity of ATM gene mutations detected in patients with ataxia-telangiectasia

The ataxia-telangiectasia mutated (ATM) gene, which is mutated in the autosomal recessive disorder ataxia-telangiectasia (AT), was isolated in 1995 by positional cloning. Although in vitro cell fusion studies had suggested that AT was genetically heterogeneous, all AT patients studied to date have been found to harbor mutations in the ATM gene. More that 100 ATM mutations occurring in AT patients have been documented. The mutations are broadly distributed throughout the ATM gene. Except for patients from families with known consanguinity, most AT patients are compound heterozygotes. The majority (> 70%) of mutations are predicted to lead to protein truncation. A significant number of the reported mutations affect mRNA splicing with at least half of the coding exons (32/62) having been observed to undergo exon skipping. The large size of the ATM gene, 66 exons spanning approximately 150 kb of genomic DNA, together with the diversity and broad distribution of mutations in AT patients greatly limits the utility of direct mutation screening as a diagnostic tool, or method of carrier identification, except where founder effect mutations are involved.

The Mre11 complex and ATM: collaborating to navigate S phase

Recently, findings regarding a group of cancer predisposition and chromosome instability syndromes, Nijmegen breakage syndrome (NBS), the ataxia-telangiectasia-like disorder (A-TLD) and ataxia telangiectasia have shed light on the unexpected role of recombinational DNA repair proteins in DNA-damage-dependent cell-cycle regulation. Mutations in the Mre11 complex cause A-TLD and NBS. In addition, functions of the Mre11 complex have been biochemically linked to ATM, the large protein kinase that is defective in ataxia-telangiectasia cells by the observation that Nbs1 is a bona fide substrate of the ATM kinase.

Ataxia-telangiectasia: phenotype/genotype studies of ATM protein expression, mutations, and radiosensitivity

Previous studies on a limited number of ataxia-telangiectasia (A-T) patients with detectable levels of intracellular ATM protein have suggested a genotype/phenotype correlation. We sought to elucidate this possible correlation by comparing ATM protein levels with mutation types, radiosensitivity, and clinical phenotype. In this study, Western blot analysis was used to measure ATM protein in lysates of lymphoblastoid cell lines (LCLs) from 123 unrelated A-T patients, 10 A-T heterozygotes, and 10 patients with phenotypes similar to A-T. Our Western blot protocol can detect the presence of ATM protein in as little as 1 microg of total protein; at least 25 microg of protein was tested for each individual. ATM protein was absent in 105 of the 123 patients (85%); most of these patients had truncating mutations. The remaining subset of 18 patients (15%) had reduced levels of normal-sized ATM protein; missense mutations were more common in this subset. We used a colony survival assay to characterize the phenotypic response of the LCLs to radiation exposure; patients with or without detectable ATM protein were typically radiosensitive. Nine of 10 A-T heterozygotes also had reduced expression of ATM, indicating that both alleles contribute to ATM protein production. These data suggest that although ATM-specific mRNA is abundant in A-T cells, the abnormal ATM protein is unstable and is quickly targeted for degradation. We found little correlation between level of ATM protein and the type of underlying mutation, the clinical phenotype, or the radiophenotype.

Mutations at the ataxia-telangiectasia locus and clinical phenotypes of A-T patients

Mutations at the ataxia-telangiectasia (A-T) locus on chromosome band 11q22 cause a distinctive autosomal recessive syndrome in homozygotes and predispose heterozygotes to cancer, ischemic heart disease, and early mortality. PCR amplification from genomic DNA and automated sequencing of the entire coding region (66 exons) and splice junctions detected 77 mutations (85%) in 90 A-T chromosomes. Heteroduplex analysis detected another 42 mutations at the A-T locus. Out of a total of 71 unique mutations, 50 were found only in a single family, and 51 had not been reported previously. Most (58/71, 82%) mutations were frameshift and nonsense mutations that are predicted to cause truncation of the A-T protein; the less common mutation types were missense (9/71, 13%), splicing (3/71, 4%) and one in-frame deletion, 2546 3 (1/71, 1%). The mean survival and height distribution of 134 A-T patients correlated significantly with the specific mutations present in the patients. Patients homozygous for a single truncating mutation, typically near the N-terminal end of the gene, or heterozygous for the in-frame deletion 2546 3, were shorter and had significantly shorter survival than those heterozygous for a splice site or missense mutation, or heterozygous for two truncating mutations. Alterations of the length or amino acid composition of the A-T gene product affect the A-T clinical phenotype in different ways. Mutation analysis at the A-T locus may help estimate the prognosis of A-T patients.

The ATM gene and breast cancer: is it really a risk factor?

The genetic determinants for most breast cancer cases remain elusive. Whilst mutations in BRCA1 and BRCA2 significantly contribute to familial breast cancer risk, their contribution to sporadic breast cancer is low. In such cases genes frequently altered in the general population, such as the gene mutated in Ataxia telangiectasia (AT), ATM may be important risk factors. The initial interest in studying ATM heterozygosity in breast cancer arose from the findings of epidemiological studies of AT families in which AT heterozygote women had an increased risk of breast cancer and estimations that 1% of the population are AT heterozygotes. One of the clinical features of AT patients is extreme cellular sensitivity to ionising radiation. This observation, together with the finding that a significant proportion of breast cancer patients show an exaggerated acute or late normal tissue reactions after radiotherapy, has lead to the suggestion that AT heterozygosity plays a role in radiosensitivity and breast cancer development. Loss of heterozygosity in the region of the ATM gene on chromosome 11, has been found in about 40% of sporadic breast tumours. However, screening for ATM mutations in sporadic breast cancer cases, showing or not adverse effects to radiotherapy, has not revealed the magnitude of involvement of the ATM gene expected. Their size and the use of the protein truncation test to identify mutations limit many of these studies. This latter parameter is critical as the profile of mutations in AT patients may not be representative of the ATM mutations in other diseases. The potential role of rare sequence variants within the ATM gene, sometimes reported as polymorphisms, also needs to be fully assessed in larger cohorts of breast cancer patients and controls in order to determine whether they represent cancer and/or radiation sensitivity predisposing mutations.

ATM Is Upregulated During the Mitogenic Response in Peripheral Blood Mononuclear Cells

Patients with the human genetic disorder ataxia-telangiectasia (A-T) are characterized by immunodeficiency and a predisposition to develop lymphoid malignancies. The gene mutated in A-T patients, ATM, codes for a high molecular weight protein that is implicated in DNA damage recognition and cell cycle control. The ATM protein does not change in amount or cellular distribution throughout the cell cycle or in response to DNA damaging agents. Because peripheral blood mononuclear cells (PBMCs) are largely in a state of quiescence and can be readily stimulated to enter a proliferative phase and because A-T cells exhibit growth abnormalities and senescence, indicative of a general intracellular defect in signalling, we chose PBMCs to examine the relationship of ATM to the proliferative status of the cell. We show here that ATM protein is present at low levels in freshly isolated PBMCs and increases approximately 6-fold to 10-fold in response to a mitogenic stimulus, reaching a maximum after 3 to 4 days. A similar, but delayed response, was evident in the presence of serum only. This increase in ATM protein was accompanied by an increase in ATM kinase activity. While expression of ATM protein increased during proliferation, ATM mRNA expression was unchanged in stimulated and unstimulated cells and there was no evidence for increased ATM protein stability in the phytohemagglutinin (PHA)-treated cells. In keeping with the reduced levels of ATM in quiescent cells, the extent of radiation-induction of the p53 pathway was significantly lower than in mitogen-stimulated cells. Basal levels of p21 were elevated in quiescent cells, and the response to radiation was negligible or reduced compared with proliferating cells over a 2-hour period. Overall, the data suggest that the increase in ATM protein in proliferating cells is due to posttranscriptional regulation and points to a role for ATM in more general signalling.

ATM Is Upregulated During the Mitogenic Response in Peripheral Blood Mononuclear Cells

Patients with the human genetic disorder ataxia-telangiectasia (A-T) are characterized by immunodeficiency and a predisposition to develop lymphoid malignancies. The gene mutated in A-T patients, ATM, codes for a high molecular weight protein that is implicated in DNA damage recognition and cell cycle control. The ATM protein does not change in amount or cellular distribution throughout the cell cycle or in response to DNA damaging agents. Because peripheral blood mononuclear cells (PBMCs) are largely in a state of quiescence and can be readily stimulated to enter a proliferative phase and because A-T cells exhibit growth abnormalities and senescence, indicative of a general intracellular defect in signalling, we chose PBMCs to examine the relationship of ATM to the proliferative status of the cell. We show here that ATM protein is present at low levels in freshly isolated PBMCs and increases approximately 6-fold to 10-fold in response to a mitogenic stimulus, reaching a maximum after 3 to 4 days. A similar, but delayed response, was evident in the presence of serum only. This increase in ATM protein was accompanied by an increase in ATM kinase activity. While expression of ATM protein increased during proliferation, ATM mRNA expression was unchanged in stimulated and unstimulated cells and there was no evidence for increased ATM protein stability in the phytohemagglutinin (PHA)-treated cells. In keeping with the reduced levels of ATM in quiescent cells, the extent of radiation-induction of the p53 pathway was significantly lower than in mitogen-stimulated cells. Basal levels of p21 were elevated in quiescent cells, and the response to radiation was negligible or reduced compared with proliferating cells over a 2-hour period. Overall, the data suggest that the increase in ATM protein in proliferating cells is due to posttranscriptional regulation and points to a role for ATM in more general signalling.

Diagnosis of ataxia telangiectasia with the glycophorin A somatic mutation assay

There are no widely applied definitive laboratory tests for the diagnosis of ataxia telangiectasia (AT). We, and others, have previously reported significantly elevated levels of in vivo somatic mutation in blood samples from known AT patients, observations that might form the basis for a useful prospective laboratory test for confirmation of a clinical diagnosis of AT. In the present case, a 4 1/2-year-old black female was suspected of having AT based on ataxic gait and chronic upper respiratory infections. Blood work-up showed low IgG2 and elevated alpha-fetoprotein (AFP), consistent with the AT phenotype. Her peripheral blood karyotype was normal, however, with no spontaneous breakage observed among 100 solid stained metaphases. Lymphocytes from AT patients often show elevated levels of chromosome rearrangement, especially at sites of immunoglobulin and T-cell receptor genes. Therefore, a blood sample was analyzed with the glycophorin A (GPA) in vivo somatic mutation assay. The GPA assay detects and quantifies the phenotypically variant erythrocytes resulting from loss of heterozygosity for the MN blood group. The patient had a 10-fold increased frequency of variant erythrocytes with a phenotype consistent with simple loss of the N allele, which is characteristic of AT. In addition, the variant cell distribution for this patient showed three other, more qualitative hallmarks of AT: a normal frequency of allele loss and duplication events, a unique ridge of cells of intermediate phenotype between the normal and mutant peaks, and evidence of similar ongoing mutational loss of the M allele. Together with clinical data, these distinctive qualitative and quantitative features of the GPA assay allow for a diagnosis of AT with a projected accuracy of 95%. Therefore, we suggest that the GPA assay, which can be performed on < 1 ml of blood and completed in less than a day, be considered as a confirmatory laboratory test for a clinical diagnosis of AT.

Ataxia-telangiectasia, cancer and the pathobiology of the ATM gene

Ataxia-telangiectasia (A-T) is a pleiotropic inherited disease characterized by neurodegeneration, cancer, immunodeficiencies, radiation sensitivity, and genetic instability. Although A-T homozygotes are rare, the A-T gene may play a role in sporadic breast cancer and leukemia. ATM, the gene responsible for A-T, is homologous to several cell cycle checkpoint genes from other organisms. ATM is thought to play a crucial role in a signal transduction network that modulates cell cycle checkpoints, genetic recombination, apoptosis, and other cellular responses to DNA damage. New insights into the pathobiology of A-T have been provided by the creation of Atm-/- mice and by in vitro studies of ATM function. Analyses of ATM mutations in A-T patients and in sporadic tumors suggest the existence of two classes of ATM mutation: null mutations that lead to A-T and dominant negative missense mutations that may predispose to cancer in the heterozygous state.

Cellular responses to radiation and risk of breast cancer

Mutations in the ataxia telangiectasia gene (ATM) result in an abnormal p53-mediated cellular response to DNA damage produced by ionising radiation. This deficiency is believed to contribute to the radiosensitivity and high cancer risk seen in ataxia telangiectasia (AT) patients and AT heterozygotes. Epidemiological studies have demonstrated that relatives of AT patients are particularly predisposed to breast cancer. This observation, together with the finding that a relatively high proportion of breast cancer patients display an abnormal severe reaction of normal tissues following radiotherapy, has led to the suggestion that AT heterozygosity plays a role in radiosensitivity and breast cancer development. The cloning of the ATM gene has allowed this possibility to be examined at the molecular level. The studies reported to date remain inconclusive, with the number of AT heterozygotes being found in radiosensitive breast cancer patients being less than would be expected based on the family studies. The potential role of several other recently identified genes which are involved in the cellular DNA damage response to ionising radiation and which could also play a role in radiosensitivity and breast cancer development are reviewed.

Strategies for mutational analysis of the large multiexon ATM gene using high-density oligonucleotide arrays

Mutational analysis of large genes with complex genomic structures plays an important role in medical genetics. Technical limitations associated with current mutation screening protocols have placed increased emphasis on the development of new technologies to simplify these procedures. High-density arrays of >90,000-oligonucleotide probes, 25 nucleotides in length, were designed to screen for all possible heterozygous germ-line mutations in the 9.17-kb coding region of the ATM gene. A strategy for rapidly developing multiexon PCR amplification protocols in DNA chip-based hybridization analysis was devised and implemented in preparing target for the 62 ATM coding exons. Improved algorithms for interpreting data from two-color experiments, where reference and test samples are cohybridized to the arrays, were developed. In a blinded study, 17 of 18 distinct heterozygous and 8 of 8 distinct homozygous sequence variants in the assayed region were detected accurately along with five false-positive calls while scanning >200 kb in 22 genomic DNA samples. Of eight heterozygous sequence changes found in more than one sample, six were detected in all cases. Five previously unreported sequence changes, not found by other mutational scanning methodologies on these same samples, were detected that led to either amino acid changes or premature truncation of the ATM protein. DNA chip-based assays should play a valuable role in high throughput sequence analysis of complex genes.

Radiosensitivity and oxidative signalling in ataxia telangiectasia: an update

Radiosensitivity is a major hallmark of the human genetic disorder ataxia telangiectasia. This hypersensitivity to ionizing radiation has been demonstrated in vivo after exposure of patients to therapeutic doses of radiation and in cells in culture. Clearly an understanding of the nature of the molecular defect in ataxia telangiectasia will be of considerable assistance in delineating additional pathways that determine cellular radiosensitivity/radioresistance. Furthermore, since patients with this syndrome are also predisposed to developing a number of leukaemias and lymphomas, the possible connection between radiosensitivity and cancer predisposition is of interest. Now that the gene (ATM) responsible for this genetic disease has been cloned and identified, progress is being made in determining the role of the ATM protein in mediating the effects of cellular exposure to ionizing radiation and other forms of redox stress. Proteins such as the product of the tumour suppressor gene p53 and the proto-oncogene c-Abl (a protein tyrosine kinase) have been shown to interact with ATM. Since several intermediate steps in both the p53 and c-Abl pathways, activated by ionizing radiation, are known it will be possible to map the position of ATM in these pathways and describe its mechanism of action. What are the clinical implications of understanding the molecular basis of the defect in ataxia telangiectasia (A-T)? As outlined above, since radiosensitivity is a universal characteristic of A-T, understanding the mechanism of action of ATM will provide additional information on radiation signalling in human cells. With this information it may be possible to sensitize tumour cells to radiation and thus increase the therapeutic benefit of radiotherapy. This might involve the use of small molecules that would interfere with the normal ATM-controlled pathways and thus sensitize cells to radiation or alternatively it might involve the efficient introduction of ATM anti-sense cDNA constructs into tumours to achieve the same end-point.

Genotype-phenotype relationships in ataxia-telangiectasia and variants

Ataxia-telangiectasia (A-T) is an autosomal recessive disorder characterized by cerebellar degeneration, immunodeficiency, chromosomal instability, radiosensitivity, and cancer predisposition. A-T cells are sensitive to ionizing radiation and radiomimetic chemicals and fail to activate cell-cycle checkpoints after treatment with these agents. The responsible gene, ATM, encodes a large protein kinase with a phosphatidylinositol 3-kinase-like domain. The typical A-T phenotype is caused, in most cases, by null ATM alleles that truncate or severely destabilize the ATM protein. Rare patients with milder manifestations of the clinical or cellular characteristics of the disease have been reported and have been designated "A-T variants." A special variant form of A-T is A-TFresno, which combines a typical A-T phenotype with microcephaly and mental retardation. The possible association of these syndromes with ATM is both important for understanding their molecular basis and essential for counseling and diagnostic purposes. We quantified ATM-protein levels in six A-T variants, and we searched their ATM genes for mutations. Cell lines from these patients exhibited considerable variability in radiosensitivity while showing the typical radioresistant DNA synthesis of A-T cells. Unlike classical A-T patients, these patients exhibited 1%-17% of the normal level of ATM. The underlying ATM genotypes were either homozygous for mutations expected to produce mild phenotypes or compound heterozygotes for a mild and a severe mutation. An A-TFresno cell line was found devoid of the ATM protein and homozygous for a severe ATM mutation. We conclude that certain "A-T variant" phenotypes represent ATM mutations, including some of those without telangiectasia. Our findings extend the range of phenotypes associated with ATM mutations.

Ataxia-telangiectasia and the Nijmegen breakage syndrome: related disorders but genes apart

Gene mutations provide valuable clues to cellular metabolism. In humans such insights come mainly from genetic disorders. Ataxia-telangiectasia (A-T) and Nijmegen breakage syndrome (NBS) are two distinct but closely related, single gene disorders that highlight a complex junction of several signal transduction pathways. These pathways appear to control defense mechanisms against specific types of damage to cellular macromolecules, and probably regulate the processing of certain types of DNA damage or normal intermediates of DNA metabolism. A-T is characterized primarily by cerebellar degeneration, immunodeficiency, genome instability, clinical radiosensitivity, and cancer predisposition. NBS shares all these features except cerebellar deterioration. The cellular phenotypes of A-T and NBS are almost indistinguishable, however, and include chromosomal instability, radiosensitivity, and defects in cell cycle checkpoints normally induced by ionizing radiation. The recent identification of the gene responsible for A-T, ATM, has revealed its product to be a large, constitutively expressed phosphoprotein with a carboxy-terminal region similar to the catalytic domain of phosphatidylinositol 3-kinases (PI 3-kinases). ATM is a member of a family of proteins identified in various organisms, which share the PI 3-kinase domain and are involved in regulation of cell cycle progression and response to genotoxic agents. Some of these proteins, most notably the DNA-dependent protein kinase, have an associated protein kinase activity, and preliminary data indicate this activity in ATM as well. Mutations in A-T patients are null alleles that truncate or destabilize the ATM protein. Atm-deficient mice recapitulate the human phenotype with slower nervous-system degeneration. Two ATM interactors, c-Abl and p53, underscore its role in cellular responses to genotoxic stress. The complexity of ATM's structure and mode of action make it a paradigm of multifaceted signal transduction proteins involved in many physiological pathways via multiple protein-protein interactions. The as yet unknown NBS protein may be a component in an ATM-based complex, with a key role in sensing and processing specific DNA damage or intermediates and signaling their presence to the cell cycle machinery.

Ataxia-telangiectasia: identification and detection of founder-effect mutations in the ATM gene in ethnic populations

To facilitate the evaluation of ATM heterozygotes for susceptibility to other diseases, such as breast cancer, we have attempted to define the most common mutations and their frequencies in ataxia-telangiectasia (A-T) homozygotes from 10 ethnic populations. Both genomic mutations and their effects on cDNA were characterized. Protein-truncation testing of the entire ATM cDNA detected 92 (66%) truncating mutations in 140 mutant alleles screened. The haplotyping of patients with identical mutations indicates that almost all of these represent common ancestry and that very few spontaneously recurring ATM mutations exist. Assays requiring minimal amounts of genomic DNA were designed to allow rapid screening for common ethnic mutations. These rapid assays detected mutations in 76% of Costa Rican patients (3), 50% of Norwegian patients (1), 25% of Polish patients (4), and 14% of Italian patients (1), as well as in patients of Amish/Mennonite and Irish English backgrounds. Additional mutations were observed in Japanese, Utah Mormon, and African American patients. These assays should facilitate screening for A-T heterozygotes in the populations studied.

Interaction between ATM protein and c-Abl in response to DNA damage

The gene mutated in the autosomal recessive disorder ataxia telangiectasia (AT), designated ATM (for 'AT mutated'), is a member of a family of phosphatidylinositol-3-kinase-like enzymes that are involved in cell-cycle control, meiotic recombination, telomere length monitoring and DNA-damage response. Previous results have demonstrated that AT cells are hypersensitive to ionizing radiation and are defective at the G1/S checkpoint after radiation damage. Because cells lacking the protein tyrosine kinase c-Abl are also defective in radiation-induced G1 arrest, we investigated the possibility that ATM might interact with c-Abl in response to radiation damage. Here we show that ATM binds c-Abl constitutively in control cells but not in AT cells. Our results demonstrate that the SH3 domain of c-Abl interacts with a DPAPNPPHFP motif (residues 1,373-1,382) of ATM. The results also reveal that radiation-induction of c-Abl tyrosine kinase activity is diminished in AT cells. These findings indicate that ATM is involved in the activation of c-Abl by DNA damage and this interaction may in part mediate radiation-induced G1 arrest.